A fuel cell stack has a structure comprising pluralities of stacked fuel cell units (cells) 1, each of which comprises a membrane electrode assembly (electrode structure) 2 constituted by an electrolyte membrane 201 and catalytic electrodes 202 formed on both surfaces thereof, and a pair of separators 4, 4 disposed on both sides of the membrane electrode assembly 2 via a gas diffusion layer (not shown) such as a carbon paper, etc. as shown in FIG. 22. One separator 4 is provided with fuel (hydrogen) gas-flowing grooves on a surface opposing the electrode structure 2, and the other separator 4 is provided with air-flowing grooves on a surface opposing the electrode structure 2. Each separator 4 is also provided on a periphery thereof with a projection terminal 121 serving as a terminal for outputting cell voltage, which is connected to a voltage-measuring apparatus attached to the fuel cell stack. To determine whether or not each fuel cell unit 1 constituting the fuel cell stack is under a normal operation, the voltage of each fuel cell unit 1 is measured by a voltmeter 5 disposed on a lead wire connected to a pair of separators 4, 4 arranged on both sides of each electrode structure 2 (see FIG. 23).
In a fuel cell stack of such a structure, a hydrogen gas and an oxygen gas in the air are reacted to generate electric power. Because the fuel gas remains in the fuel cell stack at the time of operation stop, power generation does not immediately stop but continues in each fuel cell unit while the remaining fuel gas and the air exist, resulting in the generation of an open circuit voltage between a pair of separators 4, 4 disposed on both sides of each electrode structure 2. Thus, working around the fuel cell stack immediately after operation stop might result in short-circuiting or electric shock.
Also, if the fuel cell stack is left to stand in a state in which about 1 V of an open circuit voltage exists per a unit cell, the particle size of a catalyst on a surface of the electrolyte membrane 201 would increase, and members constituting the fuel cell stack, for instance, metal or carbon separators would be corroded. For instance, in the case of a separator made of a metal such as stainless steel, etc., each separator may be formed by as thin a pressed plate as about 0.1 mm to decrease the laminate thickness of the overall fuel cell stack. In such a case, corrosion due to the above open circuit voltage may form penetrating pores in the separator.
On the other hand, in the case of start at such low temperatures as a freezing point or lower, the open circuit voltage becomes extremely high when a gas is introduced. In the case of start at −30° C., for instance, 1.35 V of an open circuit voltage may be generated, because the electrolyte membrane 201 is dry. Once current flows in that state, the electrolyte membrane 201 becomes a water-containing state, resulting in decrease in the open circuit voltage to nearly 1 V.
As described above, because the generation of an extremely high open circuit voltage is inevitable, it is necessary for an electric circuit to have high breakdown voltage to resist such open circuit voltage, resulting in increase in the cost of a fuel cell system accordingly.
To solve the above problem, there is a method of purging a fuel gas remaining in the fuel cell stack after the operation stop by an inert gas. Because a nitrogen gas is usually used as an inert gas, a tank for an inert gas is necessary to carry out this method. In automobiles, etc., however, not only a space for a tank for an inert gas is needed, but also there are problems of controlling the amount of the inert gas stored in the tank and its supply, making the overall fuel cell system complicated. Accordingly, purge with an inert gas is available only in an experiment fuel cell stack, and its practical use is difficult.
There is also a method of connecting resistors to terminals on both sides of the fuel cell stack and causing current to flow therethrough so that a gas remaining in the fuel cell stack is consumed to lower the open circuit voltage. In this case, the resistors are series-connected to pluralities of fuel cell units. However, the amount of the remaining fuel gas is not necessarily the same from one fuel cell unit to another, but often different. Accordingly, when current is caused to flow via the resistors connected to the fuel cell units, a reverse voltage is applied to fuel cell units, in which a fuel gas remains in small amounts and thus is consumed at higher speeds, resulting in the likelihood of damage to the fuel cell units.